Time-dependent modelling of a mountain front retreat due to a fold-to-fault controlled lateral spreading

Abstract

The Eastern Betic Cordillera (Spain) exemplifies at the Sierra de Aitana an impressive case of lateral spreading which involves a mountain anticline ridge dislodged by a post-orogenic extensional tectonic. The deep trenches testify the ongoing gravitational slope deformation featured by a rock mass lateral spreading. The Sierra de Aitana anticline ridge was dislodged along a normal fault zone which generated a scarp with a kilometric lateral continuity. The lateral spreading originated because Eocene limestone overlays Eocene marls, inducing a continuous evolution over time due to visco-plastic deformations and isolating huge prismatic blocks. To quantify the creep-driven stress-strain effects at Sierra de Aitana ridge, the mechanical and rheological parameter values of the involved rocks were quantified through field and laboratory tests which allowed to constrain an engineering-geological model based on equivalent-continuum approach. A stress-strain numerical modelling aimed at back-analysing the sequential morpho-structural evolution of the Sierra de Aitana anticline-ridge was performed by reproducing the retreat of the ridge front and the time succession of faults (re)activations. A time-dependent solution was approached by assuming a time-dependent creep configuration of the modelling and a parametric solution adopted to calibrate rock mass rheology, also taking into account: tectonic displacement due to the Aitana normal fault system by simulating these element as interface in the discretized domain; influence of pre-existing joint sets by assuming a main anisotropy in the elasto-plastic failure solution; admissible variations of the regional stress-field related to compressive and extensional phases and a generalised visco-plastic behaviour representing the ductile marls. The numerical modelling outputs that the lateral spreading mainly evolved as a creep-driven process while stress-release induced by tectonic activity is not sufficient to justify the observed landforms. The obtained results highlight a not negligible role of inherited structural elements for regulating the time evolution of the ongoing gravitational process at the Sierra the Aitana anticline.